The scientists looked at sightings of sperm whales beached in the North Sea between 1712 and 2003.

They then compared this record with astronomers' observations of sunspots, an indicator of solar radiation.

More whale strandings occurred when the Sun's activity was high, they found.

The Sun experiences cycles of activity that range from eight to 17 years, with 11 years being the average.

Short cycles are linked with periods of high-energy output, while long cycles are believed to be low energy.

Changes in levels of solar radiation have a big effect on Earth's magnetic field.

The most notable events are discharges of highly-charged particles or solar flares. These cause shimmering lights, called aurorae, in the magnetic fields in polar regions.

Big solar flares can also disrupt telecommunications and power lines and knock out delicate electronic circuitry on satellites.

Sun cycles

The researchers found that of the 97 stranding events reported around the coastal countries of the North Sea over the 291 years, 90% occurred when the Sun cycle was shorter than average.

The researchers speculate that whales may have a magnetic sense of orientation like pigeons, which are believed to navigate thanks to small magnetic crystals on their beaks.

What could happen is that male sperm whales, following a migratory path from the Norwegian Sea, could become disorientated by minor changes in the geomagnetic field as they enter the North Sea, the researchers suggest.

"In this shallow-shelf sea with a contourless seabed, often with soft bottom sediments, their deep-water sonar and other adaptations to their normal habitat may not function properly," they say.

Messing with sonar?

Previous studies have suggested that powerful marine sonar could be to blame for whale strandings by messing up the whales' sense of direction and depth.

An October 2003 study by British and Spanish marine pathologists, based on autopsies carried out on 10 beaked whales that beached in the Canary Islands a few hours after a Spanish-led naval task force passed by, found that the cetaceans had suffered a mortal attack of the "bends".

Tissue dissection showed that the whales' livers and other internal organs were filled with gas bubbles, and smaller blood vessels had been literally blown apart from inside.

The injuries were consistent with decompression sickness, in which nitrogen gas, absorbed into the blood stream, expands quickly as a submerged mammal rises to the surface, forming bubbles that can clot or breach blood vessels.